the latter is negative. In his "Theory of Tolerance and Principle 

 of Limiting Factors," Good emphasizes that plant distribution 

 is controlled primarily by climatic factors, secondarily by edaphic 

 factors; that plant processes are limited by definite ranges of in- 

 tensity of climatic and edaphic factors; and that the range of 

 amplitude for a particular factor often differs for various stages 

 of the life cycle. ^^ 



The ecological amplitude of a species depends upon the genetic 

 variability, or the array of biotypes and ecotypes that it possesses, 

 as well as upon the range of phenotypic expression of the biotypes 

 and ecotypes. For example, Stipa spartea owes its range of ecolog- 

 ical amplitude to broad phenotypic expression of one or a few 

 kinds of genotypes, but Andropogon scopanus owes its wide range 

 to many kinds of ecotypes, each one with its own individual range 

 of phenotypic expression. '^^ The ecological amplitude of many 

 species is apparently not only wide enough for it to live in its usual 

 habitat, but also to live if necessary in habitats that are less favor- 

 able. This reserve of amplitude is similar to Nicholson's idea of 

 "hyperadaptation" produced by natural selection' ^° and to 

 McMillan's "fund of genetic insurance . . . which would enable 

 survival of a population under changed habitat conditions." ''*" 

 The survival in a somewhat different environment from the orig- 

 inal one may provide a base for further ecological adaptation. On 

 the other hand, while the amplitude may be wide enough for sur- 

 vival in the physical conditions of the new habitat, competition 

 with other organisms better adapted to this environment may 

 prevent survival. 



The ecological amplitude of a species is often decisive in deter- 

 mining whether or not it will be present in a certain habitat or 

 community. It is also often decisive in determining the endurance 

 of a species to fluctuations in the environment within a certain 

 habitat. In the long course of evolution some species have become 

 attuned to the amplitudes of cold regions, others to those of hot 

 regions, saline deserts, or temperate forests. At least one, but 

 usually more, environmental factors may be critical in limiting 

 the ecological amplitude of a species so that it is restricted in its 

 geographical distribution. Kentucky bluegrass {Poa pratensis), for 



Ecological CHaracteristics of Species & Populations 



15 



